Inside there are several lenses- some glued together. Type of glue affects light transmission- as does quality of glass, of coatings on glass, number of pieces of glass light passes thru, etc etc etc etc.

In GENERAL- field of view is a function of the optical focus. As power goes up, field of view decreases. I have a couple of high mag varmint scopes you would NOT want to use for deer hunting.

Larger diameter tube permits more light to pass thru- all other things being equal.

Having said that, one of the best indicators is weight.

The lighter your wallet after paying for it, the better. I have a REAL cheap Chinese 60X spotter scope. So damned dim I could view a solar eclipse thru it. I do have an older Redfield Fullfield (image looks like TV screen) and a Millet Buck Silver that were not real high $$$ and are decent- but usually- money talks.

Depends on what you are scoping- a .22 for 50 yd targets, a .17 HMR for squirrels, a .204 for groundhogs, or a .50BMG for light vehicles at a mile.

Many cheap scopes use a big objective to increase light in compensation for the lower quality glass, adhesives, and coatings(or lack of coatings) used to keep manufacturing costs down. That is why you can get a Barska, NC Star scope with high magnification and a huge objective lens for less money than you could buy a Burris, Schmidt and Bender, or other high quality scope of lower power and smaller objective. Often the high quality scope with less power and smaller objective will still give a brighter picture, and better optical clarity and resolution.

I keep seeing reference to "all things being equal" while in fact if all thing are actually equal, a larger tube does nothing.

The more important factor is the size of the lenses in the erector assembly (the lens that correct or "erect" the image to view it correctly instead of inverted). The same size erector assembly in a large tube will allow for more adjustment not more light.
Quality/number of the lenses (yes it can vary) and the amount of brightness or contrast DESIGNED into the scope is what matters and only the individual user can determine if it is bright enough. Two people can have very different experiences with the same scope.

Something also that helps with Light Transmission in a Scope, is the number and type of Coatings put onto each Lens.
Single Coating, Multi-Coating, Fully Multi-Coated.
Some Manufacturer's say the Color of Coatings help Transmit Light, which is why you will see Scopes with Red, Blue, Green Lens.
I mostly think this is a Marketing Scheme, but I guess a certain Color may help with Dusk/Dawn Colors to see better. Kinda like Yellow is used for Night Time Driving to fool the Eyes with the Color Spectrum to see better.
Don't know if the Gases used is beneficial in Light Transmission or Seal Protection, but there sure is a push nowadays in the use of Aragon over Nitrogen in the higher priced Scopes.
Something that Tch2Fly mentioned about the Erector Assembly, a Matched Lens Set will also make for a better, highter cost, quality Scope in Clarity/Light Transmission over someone just randomly picking a Lens out of a box of couple thousand Lens.
Del

The more lenses you have the less light gets through. That's where the quality of glass and coatings come in. The brightest scopes are the simplest ones made of high quality glass. Its a major reason the soviets stuck with the pu type scope and the pe type faded out. Easier to produce in large quantity with a huge field of view using cheaper easier to make glass that didn't need coatings to help light pass through.

Coatings have to be high quality reduce glare and help light pass through the glass. Not all coatings are quality just because the glass is coated. A lot of scope makers use coatings as buzzwords to sell scopes, because most folks don't know what they are looking at.

When you grind glass into a curve you get a fish eye effect at the edges. There is nothing you can do about that except reduce the curve and add more lenses to make up for that. Adding lenses also reduces light transmission. The other thing that can be done and is often done in is hide the edges of the glass reducing field of view to "remove" the edge blur.

Some companies like trijicon do not hide the edges in order to keep a wide field of view (my left and one reason I like trijicon) they use superior coatings and excellent glass. This gives a big fov great light transmission for a super sharp image in low light. If your a hunter that's huge.

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